This invention relates to hoisting devices for concrete slabs and, more particularly, to a hoisting device that can be remotely disengaged and to an improved anchor structure for efficiently forming in a concrete slab a socket opening that cooperates with the hoisting device.
The known prior art relating to formation of socket openings in concrete slabs is evidenced by U.S. Pat. Nos. 3,590,538 and 3,705,469. Neither of the patents discloses a simple sleeve defining an annular abutment for engagement with a hoisting device.
Hoisting devices for concrete slabs are shown in such patents as U.S. Pat. Nos. 3,420,014; 3,431,012 and 3,652,118. The '014 and '012 patents are not adapted for remote control disengagement because they require that the entire body of the hoisting device be rotated relative to the slab. The device shown in the '118 patent is likewise incapable of affording remotely controlled disengagement because of the presence of a spreader key which is subject to extremely high frictional forces.
Removable plugs for excluding concrete from an anchor opening during placement of the concrete while in the fluid state are disclosed in such patents as U.S. Pat. Nos. 3,590,538 and 3,596,971. Those patents do not disclose a plug capable of forming a line of perforations in the concrete that covers the plug.